All 31 passengers of the two boats are safe and have been successfully brought to shore.
Officials from Hat Noppharat Thara–Mu Ko Phi Phi National Park said the boat operator, identified as 37-year-old Apichart Kobtaisong, had taken 10 foreigners on an island tour.
He had reportedly slowed down his vessel as it approached Chicken Island so the tourists could take photos of the spectacular sunset. At that point, his boat was hit by another longtail boat and he was knocked overboard.
Apichart was still missing as of press time.
Yutthapong Dumsrisuk, chief of the National Park, said the authorities had conducted joint operations to locate Apichart within a 10-kilometre area of the collision, using drones, boats and divers all day on Saturday, but in vain. He said he would call on local residents and other tour agencies to also help search for Apichart.